The present invention relates to a digital signal transmission system for effecting data transmission in a time division multiplex mode by using a communication cable, and particularly relates to a digital signal transmission system in which the sending timing of a packet signal onto the communication cable from each station is regulated in accordance with the guard time.
As the use of computers has spread and as digital signal processing techniques have progressed, a data communication technique has moved into the limelight in which a communication system and a data processing system are combined so as to enable information to be processed by on-line processing. As a small-scale communication system, such as a private communication system installed in the precincts of government and public agencies, companies, or the like, such a communication system in a packet form using a communication cable, e.g. a coaxial cable, attracts public attention due to its low cost, high reliability and high transmission efficiency.
In such a packet-form communication system, a number of personal stations are connected to a communication cable for effecting bi-directional transmission to and from a computer disposed in a laboratory or the like so that messages each divided into data blocks of 1,000 to 2,000 bits may be transmitted from each station. Each message is additionally provided with a header containing its designation, running number or the like. In this communication system, control functions are completely distributed to the respective stations and therefore the network per se is a mere passive transmitting medium having no control function. Accordingly, each station begins transmitting a message after it confirms that the transmission line is available when interference with a packet from another station occurs during the transmitting operation, both the concerned stations stop their transmitting operations. Each of the stations which has stopped its transmitting operation will then try to transmit the message again after a random queuing time.
In such a communication system, any user at any station not only can access one and the same computer but also can utilize any hardware such as a memory or any software such as a program among the hardware and software distributed amongst the plurality of stations. That is, in this communication system, devices such as high speed or high accuracy printers, large scale files, or the like, which have been concentrated at the location of a large central computer in a time sharing system, may be utilized substantially equally by all stations. Thus, it becomes possible not only to economize resources and to improve practical efficiencies but also to develop a large-scaled software system due to the accommodation of programs and data. Further, in such a communication system, there is no priority in using the transmission line among the users or personal stations. Accordingly, there is no master and slave relationship which is often provided in other systems, so that communication may be carried on between any among the connected stations. Further, since the transmission line such as a coaxial cable is constituted by a complete passive circuit, a highly reliable system may be easily provided.
While this communication system has various advantages, but there is a possibility in this system that packets will interfere with each other on the same transmission line since each station may begin transmitting data at any time. Such interference between packets will become significant as the operating efficiency of the transmission line becomes higher.
To solve such a problem, there have been proposed a number of signal transmission systems such as the so-called "priority Ethernet" and "Reservation Ethernet" Systems. In the former system, the priority of signal transmission of each station is indicated in the preamble portion of the packet so that, in case interference occurs between packets from different stations, one of the packets having higher priority is allowed to be transmitted preferentially. In the latter system, a master station which indicates the operation mode is always set so as to confirm whether each of the other, personal stations has a signal in a reserved mode waiting to be transmitted and the amount of information to be transmitted. As a result, the master station determines in every frame the order of packets to be transmitted by the respective stations so as to allow signals to be transmitted in time division multiplex in the transmitting operation mode.
In the former proposed signal transmission system, however, there is still a problem of variations in signal transmission delay time due to interference among packets having the same priority. Accordingly, this system is not suitable for real time transmission, such as conversational sound communication, in which importance is attached to the real time correspondency between transmitting and receiving operations.
In the latter signal transmission system, however, the above-mentioned inter-station equality is lost because of the existence of the master station. That is, in this system, data communication must be stopped if any failure occurs in the master station, and in this sense the system reliability suffers.
In order to solve this problem, there has been proposed a digital signal transmission system in which real time transmission can be effected without losing the equality among personal stations. In this system, a frame which is cyclically repeated along the time axis is subdivided on the same time axis into a plurality of blocks so that each personal station may be given an opportunity for packet communication within the block. Thus, each station not only may have an equal opportunity to use an empty block but can also effect real time transmission because an opportunity for signal transmission is given periodically in every frame if the station occupies a certain block for a long enough period of time for the signal transmission.
FIG. 1 shows the frame configuration used in the system as mentioned directly above. A frame cyclically repeated on the time axis is constituted by N blocks #1 to #N. Each block is constituted by various bit strings b.sub.1 to b.sub.9 as follows:
b.sub.1 . . . backward guard time; PA1 b.sub.2 . . . preamble; PA1 b.sub.3 . . . start flag; PA1 b.sub.4 . . . address bit string; PA1 b.sub.5 . . . control bit string; PA1 b.sub.6 . . . information bit string; PA1 b.sub.7 . . . check bit string; PA1 b.sub.8 . . . end flag; and PA1 b.sub.9 . . . forward guard time.
The bit strings b.sub.2 to b.sub.5 and b.sub.6 to b.sub.8 are necessary to constitute a packet and are generally referred to as overhead or additional bits. Intervals b1 and b.sub.9 are generally referred to as guard time. That is, the guard time is an empty string for avoiding the situation that adjacent packets overlap with each other due to the delay time which may occur when the packets of each block propagate on the coaxial cable. The backward guard time b.sub.1 is for protecting the rear packet from such an overlap situation, while the forward guard time b.sub.9 is for protecting the forward packet in the same manner. The number of total bits of the backward guard time b.sub.1 and the forward guard time b.sub.9 is represented by g and the guard time (b.sub.1 +b.sub.2) is represented by .tau..sub.g.
In this proposed digital signal transmission system, if no station is sending signals, any station can begin to send out such a frame configuration signal as described above at any time. A station which has first begun to send out a signal onto the communication cable takes the initiative of frame synchronization.
Once the frame synchronization has been established in this manner, all stations can monitor the status of signals transmitted on the communication cable. The user equipment at each station is provided with a memory for indicating the occupation status of the respective blocks in every frame so that the respective blocks are registered in accordance with the received packet signal of each station. When another station sends out a packet signal after the frame synchronization has been established, the station first searches for an empty block in accordance with the contents of the memory, occupies the block to prevent other stations from transmitting in that block, and times its own with the thus occupied block.
In this case, however, the timing for the initiation of signal transmission becomes a problem. For example, as shown in FIG. 2, assume that a station C is located at a longitudinally central portion of coaxial cable 3 which is connected at its opposite ends to impedance matching terminators 1 and 2, and another station S located between the station C and the terminator 1 is now transmitting. In this case, the packet signal sent out from the station S may be received by the station C and further stations R.sub.1 and R.sub.4 on the coaxial cable 3 at different points of time depending on the signal propagation delay time on the cable. Accordingly, if each station sends out its own signal with no consideration for this delay time, there may develop a situation wherein adjacent packets on the coaxial cable 3 overlap with each other.
To prevent such a serious situation from occurring, this proposed system utilizes the concept of above-mentioned guard time .tau..sub.g. That is, the guard time .tau..sub.g in this system is set to be equal to or more than twice the signal propagation delay time between the central station C, which is regarded as a positional reference, and the farthest station, and signal transmission from each station is controlled such that the packet signals transmitted from the respective stations may be arranged equidistantly in a row at the receiving position of the station C.
This feature will be more particularly described by referring to FIG. 3. Assume now that the station S has completed its signal transmission and other stations R1 to R4 are going to begin transmitting their packet signals. In this case, each of the succeeding stations R.sub.1 to R.sub.4 determines the timing of its own packet signal transmission such that its transmitted packet signal will be received at the reference station C at a point in time which is one guard time after the station C completes its reception of the packet signal transmitted from the preceding station S (transmitted S packet). For the determination of the signal transmission timing, each station considers the positional relationship on the cable among all the stations connected to the cable. When the packet signal transmitted on the cable is received by a station, the station identifies the fact that the packet received is from the station S (received S packet) on the basis of the address bit of the received S packet, and determines the time when the received S packet will terminate at the station C on the basis of the positional relationship between the location of the station and the location of the reference station C and also on the basis of signal propagation delay time between the same two stations. The received S packet ending time at the station C will be later than that at the station R.sub.1 and R.sub.2 and earlier than that at the stations R.sub.3 and R.sub.4.
When the received S packet ending time with respect to the reference station C has been determined by each of the succeeding stations R.sub.1 to R.sub.4, any one of the succeeding stations R.sub.1 to R.sub.4 which desires to send out its own signal begins to send out a packet signal (transmitting R packet) at a time earlier than the abovementioned received S packet ending time at the station C by the signal propagation delay time from the one station to the station C. The reception of the packet signal sent out in this manner (receiving R packet) will begin at the station C at a point in time which is later than the receiving S packet ending time by the guard time .tau..sub.g.
The signal transmission timing is controlled by establishing frame synchronization and block synchronization. That is, each station periodically resets with predetermined timings both a block counter and a frame counter for counting clock signals produced by an intra-office clock generator, thereby establishing frame synchronization as well as block synchronization within the error range of the clock signal frequency.
For example, at the station R.sub.3 which is disposed beyond the station C when viewed from the station S, the above-mentioned counters are controlled such that the second block #2 is initiated at a time later than the receiving S packet ending time by a period of time .tau..sub.b9 (which is referred to as "forward guard time") which can be calculated by the following equation (1) EQU .tau..sub.b9 =.tau..sub.g /2-.tau..sub.CR3 ( 1)
where .tau..sub.CR3 represents the signal propagation delay time between the station C and the station R.sub.3.
For the station R.sub.2 which is located between the station S and the station C, the forward guard time .tau..sub.b9 is expressed by the following equation (2): EQU .tau..sub.b9 =.tau..sub.g /2+.tau..sub.CR2 ( 2)
where .tau..sub.CR2 represents the signal propagation delay time between the station C and the station R.sub.2.
Similarly, at the station R.sub.1 which is located beyond the station S when viewed from the station C, the forward guard time .tau..sub.b9 is expressed by the following equation (3): EQU .tau..sub.b9 =.tau..sub.g /2+.tau..sub.SC -.tau..sub.SR1 ( 3)
where .tau..sub.SC and .tau..sub.SR1 represent the respective signal propagation delay times between the stations S and C and between the stations S and R.sub.1.
Thus, in the previously proposed digital signal transmission system, it has been necessary to determine forward guard time which is the time lapse from the completion of reception of the packet (herein after referred to as a "master packet") sent out from a station taking the initiative for frame synchronization to the commencing of the next block, on the basis of a selected one of the above-mentioned equations (1) to (3). In each station, the forward guard time .tau..sub.b9 determined in accordance with the relationship with respect to other stations is stored in memory means such as a read only memory (ROM). Each station can obtain a signal corresponding to the forward guard time .tau..sub.b9 from the output terminal of the thus prepared memory means by applying the designation, or identity, of the master packet to the address terminal of the memory means. The thus obtained forward guard time signal is then applied to a programmable timer, for example, the output of which then resets the above-mentioned counters so as to establish the frame synchronization and block synchronization. The programmable timer is a general purpose circuit which may be started when it is applied with the above-mentioned signal corresponding to the forward guard time .tau..sub.b9 and which may then indicate, by the rising or falling edge of its output signal, the lapse of a forward guard time from the started point of time.
In this proposed system, however, the contents of the translation tables are not only different one from another in the respective stations but also increase dramatically as the number of stations increases. Accordingly, the design and/or the production of the memory means such as a ROM for storing the translation table requires many steps, resulting in a serious deterrent to practical use of a communication system to which the proposed system is applied.